The adhesion of coatings applied directly to the surface of a substrate metal is of special concern when the coated metal will be utilized in a rigorous industrial environment. Careful attention is usually paid to surface treatment and pre-treatment operation prior to coating. Achievement particularly of a clean surface is a priority sought in such treatment or pre-treatment operation.
Representative of a coating applied directly to a base metal is an electrocatalytic coating, often containing a precious metal from the platinum metal group, and applied directly onto a metal such as a valve metal. Within this technical area of electrocatalytic coatings applied to a base metal, the metal may be simply cleaned to give a very smooth surface. U.S. Pat. No. 4,797,182. Treatment with fluorine compounds may produce a smooth surface. U.S. Pat. No. 3,864,163. Cleaning might include chemical degreasing, electrolytic degreasing or treatment with an oxidizing acid. U.S. Pat. No. 3,864,163.
Cleaning can be followed by mechanical roughening to prepare a surface for coating. U.S. Pat. No. 3,778,307. If the mechanical treatment is sandblasting, such may be followed by etching. U.S. Pat. No. 3,878,083. Or such may be followed by flame spray application of a fine-particle mixture of metal powders. U.S. Pat. No. 4,849,085.
Another procedure for anchoring the fresh coating to the substrate, that has found utility in the application of an electrocatalytic coating to a valve metal, is to provide a porous oxide layer which can be formed on the base metal. For example, titanium oxide can be flame or plasma sprayed onto substrate metal before application of electrochemically active substance, as disclosed in U.S. Pat. Nos. 4,140,813 and 4,331,528. Or the thermally sprayed material may consist of a metal oxide or nitride or so forth, to which electrocatalytically active particles have been pre-applied, as taught in U.S. Pat. No. 4,392,927.
It has, however, been found difficult to provide long-lived coated metal articles for serving in the most rugged commercial environments, e.g., oxygen evolving anodes for use in the present-day commercial application utilized in electrogalvanizing, electrotinning, electroforming or electrowinning. Such may be continuous operation. They can involve severe conditions including potential surface damage. It would be most desirable to provide coated metal substrates to serve as electrodes in such operation, exhibiting extended stable operation while preserving excellent coating adhesion. It would also be highly desirable to provide such an electrode not only from fresh metal but also from recoated metal.